I'm working on a surface mount training kit (Elenco SM-200K). One of the surprises I'm finding is that soldering to a large pad is more difficult than soldering to a small pad. That's because it takes more energy to heat up a large pad than a small pad. That makes it more difficult to get the molten solder to adhere to a large pad than to a small pad.

How do you deal with this?

I have three different types of solder: a roll of 63-37 mildly activated rosin solder of around #26 gauge (which I expected to be best for surface mount), a roll of 60-40 solder of around #20 gauge (what I was using in the past for through-hole soldering), and the roll of lead-free solder (around #20 or #22 gauge, which came with the kit).

I'm finding that the 63-37 solder is the most difficult to work with when soldering to large pads. I thought that eutectic solder is best because it avoids that solid/liquid transition phase Then again, Isuspect that the solder content isn't the issue but the solder size. I'm finding that the larger the pad, the more solder I need, as if I'm relying on the molten solder rather than the soldering iron tip toheat the pad. The 60-40 solder and lead-free solder actually adhere better simply because the larger quantity (due to the larger size) heat the pad better.

I'd like to hear from those of you who are experienced with surface mount soldering.

I use a temperature controlled soldering station set to about 800 degrees. Put liquid flux on the pad and then tin it with a light coating of solder. Then place the IC and heat the pads. This worked for some motor control ICs that required soldering heat sink tabs to large PCB ground planes.

I use a temperature controlled soldering station set to about 800 degrees. Put liquid flux on the pad and then tin it with a light coating of solder. Then place the IC and heat the pads. This worked for some motor control ICs that required soldering heat sink tabs to large PCB ground planes.

The solder already comes with flux in it. Or do these large pads simply require more flux than what the solder provides?

I use liquid rosin flux (in addition to the solder flux) for all surface mount pads. First, it helps hold the IC in place while I tack down a couple of pads. I find that the amount of flux in the solder is not sufficient for surface mount. Its fine for thru-hole devices. The little needle dispensers tend to plug if you don't use them every day so I'm now using GC 10-4202 which comes in a small bottle with a brush on the cap.

After all the surface mounted components are in place I clean the excess flux with Chemtronics ES1035 Flux-Off in a spray can.

Liquid flux is a big help . While you can use a good temp-controlled iron for occasional surface-mount work , if you plan to do much of it , you'll want an air pencil . It makes a world of difference . Also , as an aid to air soldering , 3M's Kaptan is an amber heat-resistant tape that does an absolutely incredible job of keeping heat off of adjacent components .

Temp control irons fight what you're trying to do. The problem is they limit the BTUs you can store up in the tip. If you can, turn it up as high as possible. With the ferromagnetic type irons, get a numbe r'8' tip.

Also, don't yeild to the temptation to use a small tip to put down the parts. I was using a 1/64 tip that I used to mount CPUs, and it just didn't work when I had to mount caps on a stripline antenna that didn't have 'thermals' around the pad positions. And I was using RoHS solder that needs a bit more heat. What I found was to go back to a stubby fat tip. Not too big, but something with more angle to the 'cone' of the tip in my Weller station. I even went back to a number '7' for temp.

With the caps, I found that the best way is to put a 'film' of solder down on ONE side only. Put the chipcap on it. Then tack it down. It's not really secure but it should hold. Then go to the 'clean' side and 'roll' the tip in with the solder. As it melts, it should wet both parts as long as you keep your tip rolling against both the base (the copper) and the vertical (the end of the chipcap). Especially with lead free, it seems the flux is at it's best when it FIRST melts. Then go back and put down the proper solder bead on the other side.

Don't put down too much solder!! Just enough to wet both the pad and the part, and have a nice 'web' or rivulet.

I have four heat sources I use for soldering, a small Antek 15 watt iron with a small tip, a Weller 40 watt iron for jobs where I need it, a Weller gun that I use most of the time for single or quick jobs, and a Benz-O-Matic massive iron attachment that fits over a pencil flame burner on my propane hand torch.

For surface mount, I usually use the Antek and 60-40 solder of the leaded variety. I've bought more rosin core lead solder than I'll probably use in my lifetime when talk of the ban started going. I'm not going to let scare tactics stop my use of good solder until they come out with something that is as good in quality as old fashioned solder is.

For big areas you need a big iron! I have an American Beauty 250 W. soldering iron from my days as a stained glass craftsman. That sucker can solder onto anything. The secret is the huge tip and heating element: larger mass holds more heat.

If you're only going to be doing occasional large area soldering then having a big iron may not be a priority.

So how many surface mount components have you installed with that 250W iron? :-)

I'm not suggesting that a temperature controlled soldering station should be used for stained glass work. I'm suggesting that it works well for occassional work with surface mount and it works great for most PCB work. Now if you are going to do production surface mount work then you probably want to invest in a hot air device.

Eutectic solders have many different properties that can work for or against you. Ideally an eutectic solder would melt at a low temperature, wet and flow easily, cool to a mechanically and electrically sound connection and not suffer from "tin whiskers".

Tin whiskers caused the failure of the Galaxy IV satellite. Zinc, aluminum, cadmium, gold and silver also can grow whiskers that may lead to short circuits.

With the removal of lead from solders for RoHS compliance it required every soldering formulation to change. Some formulations are almost as good as tin/lead alloys but some are pretty darned bad. If the formulation is not right on the nose you can get a soldering joint that has a granular appearance as certain elements solidify quicker than others. For one example, the amount of copper in a connection will have a noticeable physical difference with changes of .1% in the alloy.

I have some references if you wish to get really obsessive with soldering;

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